Aguero hasn't had a major injury since 2015, and he scored 15 in his last 17 games.
20 league goals last season despite being pretty crap for 6 months of a 9 month season, during which he scored only 6 league goals in 15 league games and he was suspended for another 4.
Now he's worked out what Pep wants and how to do it, breaking the goal scoring record of 31 is very much in reach, especially with the supporting cast he'll have around him.
Rotation, injuries etc - It's a gamble obviously, but that's why the odds are 50/1.